Such a system is particularly useful in a moving vehicle passing through an area with traffic problems where information on road conditions or instructions to drivers are broadcast from a control post. Thus it is known to provide automobile radios with automatic changeover means for switching from an entertainment program to special message signals transmitted for the purpose of improving road traffic. The switchover occurs as soon as such message signals are picked up by the vehicular radio receiver, as when the car enters an area within reach of the transmitter at the control post.
If such a vehicle happens to be stationary while receiving the special signals in lieu of the normal radio program, the occupants may become tired or annoyed by a recurrent message (e.g. one played from a magnetic recorder) and may simply decide to turn off the receiver. This leaves the driver unaware of the fact that messages of importance to the progress of the vehicle are still being broadcast.